WELLOGAcoustic
Log Interpretation

Acoustic logging is
also used for determination of cement bond in cased wells. This type of log is most often referred to
as a Cement Bond Log (CBL).

Acoustic signals
propagated in steel casing are observed to have large amplitude in free casing
because much of the energy is retained in the casing. Whereas the opposite
effect is found in casing that is in contact with a solid such as cement. The casing signal
is much smaller because the energy is coupled into the surrounding cement and
formation.

The thin plate velocity
of sound in steel is approximately 5300 meters per second (188 microseconds per
meter).

A receiver having 3
feet spacing will receive the casing signal (first arrival) at 174 microseconds
plus an additional period allowing for transit time thru the borehole fluid.

Consideration for fluid
travel time at approximately 200 microseconds per foot shows that for example
in a 12 inch casing, 6 inches from tool center to casing and 6 inches from
casing to tool will add 1 foot = 200 microseconds to total travel time.

Variation in total
travel time due to fluid type will cause the casing signal to move or change
position in relative time.

A receiver signal “time
gate” is set at the time of the expected casing signal. The casing signal will
be the first arrival at the receiver in free casing. The signal amplitude is
recorded.

Signal amplitude
is affected by the acoustic coupling of cement in the annulus (space between
the casing and formation). Acoustic energy is absorbed when cement is in
contact with the casing and formation.

CEMENT BOND LOG
INTERPRETATION:

In general terms,

A high signal amplitude indicates poor cement bond. A low signal amplitude indicates good cement bond. Amplitude
is normally presented on a scale of 0 to 100 percent amplitude. No cement bond
is represented by 100 percent amplitude. Due to the fact that well cemented
pipe can never reduce the signal to “zero”, a good reference for zero signal is
the best cemented portion of the cased hole. Using information obtained from a
Variable Density (waveform) display referred to as a VDL display, it is
possible to observe the entire receiver wave train. When cementation is complete
(good bond) from casing to cement to formation, it is possible to observe
waveform shift in delta- time in the later arrivals that can be correlated to
open-hole acoustic delta-time logs. This is referred to as “formation”
signal on the CBL.

CBL ATTENUATION:

The measurement of
attenuation measured in decibels (dB) is obtained from the amplitude as
follows:

Attenuation = 20/D x Log10(A/Ao)

Where:

Attenuation is measured
in decibels abbreviated dB.

Ao is the transmitter amplitude measured in millivolts

A is the receiver
amplitude measured in millivolts

D is the distance from
the transmitter to receiver (spacing)

Note: Attenuation
refers to the reduction of amplitude. Therefore, attenuation is measured in–dB.

Commercially available
CBL services offer a log containing the measurement of amplitude and VDL.

VARIABLE DENSITY LOG:

The variable density display is a Z axis
representation of the receiver waveform. The display is usually monochrome
(black and white) however in some presentations, the display is in color.

FRACTURE FINDER LOG:

A variation in
application of the VDL is in an open-hole environment that is used for the
purpose of defining fractures in surrounding formations. The service is
referred to as a fracture finder or micro-seismogram
log.

RADIAL BOND LOG:

Acoustic bond tools
have been developed that have segmented receivers. A typical eight segment
array is arranged radially around the circumference
of the tool. Each segment has a view of 45 degrees of the total 360 degree
circumference. The advantage of this configuration is that eight discrete
receivers that are able to define vertical channels in cement.

CASING INSPECTION/CEMENT
QUALITY:

Ultrasonic waveforms
having frequencies that range from 200 KHz to 700 KHz are used. A rotating acoustic
sensor is employed to measure acoustic energy through a 360 rotation. The
Ultrasonic Imager Tool (Schlumberger USIT) is an example. It performs the
functions of cement evaluation, casing inspection, corrosion detection and
monitoring, detection of internal and external damage or deformation and casing
thickness analysis for collapse and bust pressure calculations.

ACOUSTIC BOREHOLE
IMAGER:

The acoustic borehole
imager (ABI) televiewer is a tool that acoustically scans
a borehole – usually in open-hole. A very small acoustic receiver aperture is
used to increase the acoustic resolution. A stepper motor rotates the acoustic
aperture and at each step records the acoustic signature of the borehole.
Logging speeds are slow. The log is an acoustic image of the borehole that
defines structural details in a relatively high resolution format.